Drag & drop improvements

[goncalossilva]

This PR improves drag & drop by merging the best ideas from the previous DragDropManager and ItemTouchHelper into a new class with a new workflow, DragDropHelper. There's also a tiny fix to StickyHeadersLinearLayoutManager's position tracking on moves, detected while implementing it.

Reasons for not sticking with DragDropManager:

• Fairly complex
• Uses a Bitmap and not the view itself, which makes the workflow more complex and results in a ton of limitations

Reasons for not sticking with ItemTouchHelper:

• Fairly complex
• Lots of supported features, which cripples some interactions of the drag & drop itself
• Buggy when swapping fast, even on simple lists (I don't think their prepareForDrop workflow is working as well as they think...)

What does DragDropHelper bring to the table:

• Much simpler implementation than both DragDropManager (multiple classes, 220% larger) and `ItemTouchHelper (single class, but 421% larger)
• Nice callback workflow like ItemTouchHelper
• Stable and functional implementation, for simple and complex lists, like DragDropManager

What does DragDropHelper NOT bring to the table:

• Support for all built-in LayoutManagers. This would be doable, but supporting LinearLayoutManager is good enough for most cases and massively simplifies things.

Thanks for reviewing!

[rastislavvasko]

Great refactoring! 👏 Couldn't find any issues, LGTM. 👍